In closed-loop heat transfer systems known to the prior art, capillary pumping techniques have been used to supply liquid-phase working fluid to heat-exchange surfaces for evaporation thereon to vapor phase. For example, U.S. Pat. No. 4,470,450, a closed-loop heat transfer system was described, which comprises a capillary-type evaporator wherein a working fluid in liquid phase absorbs heat and is thereby evaporated to vapor phase, a condenser (preferably also of the capillary type) to which heat-laden working fluid in vapor phase is transported for condensation back to liquid phase, and a pump for returning the condensed working fluid in liquid phase from the condenser to the evaporator. However, until the present invention, two-phase circulation of a working fluid through a closed-loop heat transfer system having a capillary-type evaporator could not be precisely controlled so that liquid-phase working fluid is continuously supplied to each portion of the heat-exchange surface of the evaporator in exactly the amount needed to achieve optimally efficient heat exchange.